Broken link on Leios technical report #1

[kbennett2000]

The Leios technical report #1 (docs/technical-report-1.md) contains a broken link to "Overview for formal specification" (https://github.com/input-output-hk/ouroboros-leios/blob/main/formal-spec/docs/overview.png)

[kbennett2000]

Link to this image:
https://github.com/input-output-hk/ouroboros-leios-formal-spec/blob/main/formal-spec/docs/overview.png

[kbennett2000]

Link fixed